Graphical Interpretation of Wet Surface Evaporation Equations

Abstract

AbstractPotential (wet surface) evaporation is the basis for many methods to estimate actual evaporation. Penman's (1948, https://doi.org/10.1098/rspa.1948.0037) combination of energy budget with mass and energy transfer equations can be depicted on temperature‐vapor pressure graphs. Key to this depiction is two straight lines on the graph representing constant enthalpy of the air at measurement height and at the surface skin, with the gap between the lines representing a combination of aerodynamic resistance variables and available energy. The equations on which Penman based his formula are easily solved numerically (without need for Penman's famous assumption) for T0w, the temperature the surface would have if it was saturated, keeping all other variables constant. Wet surface evaporation is proportional to the vapor pressure difference between the measurement height and the surface skin. Equilibrium evaporation, based on the slope of the vapor pressure curve at T0w, is also easily represented in the graph. The difference between the correct wet surface evaporation rate and Penman's approximation is immediately visible on the graphs. The different wet surface evaporation rates are compared using data from a tropical savanna in Australia. Implications for the classic two‐component interpretation of Penman's equation are discussed.